The present invention relates to the synthsis of resins and more particularly to a selective resin synthesis and the resulting dual functional resins made thereby.
Vapor permeation curable coatings traditionally are a class of coatings formulated from aromatic hydroxyl-functional polymers and multi-isocyanate cross-linking agents wherein an applied film thereof is cured by exposure to a vaporous tertiary amine catalyst. In order to contain and handle the vaporous tertiary amine catalyst economically and safely, curing chambers were developed. Curing chambers typically are substantially empty boxes through which a conveyor bearing the coated substrate passes and in which the vaporous tertiary amine, normally borne by an inert gas carrier, contacts such coated substrate. The use of aromatic hydroxy-functional polymers is recommended if an extended pot life system is required. If two-pack formulations are acceptable, then use of aliphatic hydroxyl-functional resins can be made. Multi-isocyanate cross-linking agents in traditional vapor permeation curable coatings contain at least some aromatic isocyanate groups in order for practical cure rates to be achieved.
Such traditional vapor permeation curable coatings requirements have been altered to a degree by the vaporous amine catalyst spray method disclosed by Blegen in U.S. Pat. No. 4,517,222. Such vaporous catalyst spray method relies on the concurrent generation of an atomizate of a coating composition and a carrier gas bearing a catalytic amount of a vaporous tertiary amine catalyst. Such generated atomizate and vaporous catalytic amine-bearing carrier gas flow are admixed and directed onto a substrate to form a film thereover. Curing is rapid and use of a curing chamber is not required. Moreover, all aliphatic isocyanate curing agents can be utilized in such spray process. Aromatic hydroxyl groups on the resin, however, still are required.
One drawback to the requirement of aromatic hydroxyl groups on the resin is the inherent limitation which such aromaticity provides in formulating high solids coatings. The same is true of the requirement of aromaticity in the multi-isocyanate cross-linking agent. Such non-volatile solids content restriction even applies to the vaporous amine catalyst spray method described above.
Yet, despite the foregoing limitations which arise by virtue of the use of phenolic hydroxyl groups, aliphatic hydroxyl groups are not sufficiently responsive to vapor permeation cure to permit early film development. That is, a prime advantage of vapor permeation curable coating is that they rapidly develop very early film properties so that the coated part can be handled on the coatings line without fear of damage to the coating. Over the long term, aliphatic hydroxyl groups will fully cure with the polyisocyanate cross-linking agents, though extended cure times means that early handling of the coated part is lost. Thus, it would appear that a suitable blend of phenolic hydroxyl groups and aliphatic hydroxyl groups would permit early film development, yet the formulation of higher solids, and higher performance coatings.
With respect to traditional polyester polyols taught in the vapor permeation curable coatings art, partial capping (partial capping for present purposes comprehending the formation of a resinous mixture containing both phenolic hydroxyl groups and aliphatic hydroxyl groups) of polyesters sometimes can be accomplished through direct esterification of aliphatic hydroxyl-functional polyesters with phenolic-functional carboxylic acids. This technique has limited applicability since the higher reaction temperatures (about 180.degree. C. or above), requirement for acid catalysts, and long reaction times cause many side reactions to occur. Sometimes, it is a race to get the acid value down (e.g. below 10) before gelation of the reaction mixture occurs. When this technique is applied to higher functional aliphatic hydroxyl-functional acrylic resins, the resulting partially capped acrylic resinous product cannot be made reproducably on a large scale. Moreover, the esterification process produces water which must be azeotropically removed from the resinous mixture. Since such partial capping esterification has relatively slow reaction kinetics, a good azeotroping solvent (e.g. toluene or xylene) must be removed by vacuum distillation upon completion of the capping sequence. Accordingly, a more general and useful partial capping synthesis technique would be quite useful in the vapor permeation curable coatings as well as in other fields of endeavor.